How to select PWM frequency of Treadmill motor

Thread Starter

Ahmad04

Joined Feb 26, 2024
1
Hello everyone!

I am designing a PWM based motor controller circuit for a Treadmill motor (180VDC, 2.5HP). I have selected PWM frequency to be 5kHz. I have ended up blowing the MOSFET every time. I have added an RC series combination across the motor terminals which improved the MTBF of the MOSFET a bit. However, I doubt that the frequency is very high and the MOSFET is excessively heating and burns off.

I have searched over the internet to find the PWM frequency range of commercial treadmill controllers without success. If anyone out of their experience can provide me with their feedback, it would be a great favor for me.

I look forwards for your precious feedback. Furthermore, if you can suggest any other probable reason for the fault, please inform me.

Thanks.
 

Jerry-Hat-Trick

Joined Aug 31, 2022
550
There are probably people on this forum who will provide you with an expert answer, but my simplistic understanding of losses in a mosfet are that when they are conducting the losses are caused by the current flowing and the drain source resistance which can be minimized by selecting a transistor with a low RDS on, and the loss as they turn on and off over a finite time, where a good driver can help. I question why you need 5kHz PWM frequency for a motor where the system will have significant mechanical inertia and I wonder if an order of magnitude lower frequency would suffice, reducing the losses significantly. I may be wrong but worth a try?
 

MaxHeadRoom

Joined Jul 18, 2013
28,667
The popular MC2100 used 15khz on the one I measured, It is driven by a MIC4427 from the micro.
Do you have protection diodes across the Mosfets?
There is a reverse engineered schematic here if you search.
 

LowQCab

Joined Nov 6, 2012
4,052
A properly selected Choke ( Inductor ) on the Input to the Motor will solve many problems.

If You have no Choke at all,
RF-Spikes from the Motor-Brushes will probably destroy the FET, even at low Power-Levels.

An Oscilloscope will quickly reveal where the problem areas are.
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HasBeen

Joined Jul 28, 2023
29
I use a 30-amp diode across the motor winding, just like you would put across a relay coil to supress that spike. You need to have a zener diode from gate to source on the FET to limit spikes on the gate to within specs(eg, 15-volt zener for a 20-volt device). I would expect the performance to be limited at that high a PWM frequency due to needless inductive losses in the motor windings. Do you have some reason to operate at that frequency?
 

HasBeen

Joined Jul 28, 2023
29
If 20 Hz is the actual motor drive frequency, I would expect considerable chatter / pulsation from the motor. I wouldn't like that for my applications.
 

animal12

Joined Feb 20, 2018
43
simmer down ! All I said was I was " running my MC-2100 , off one of those import PWM Generators set at 20 Hz "
I will remove my self from this thread now .
animal12
 
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